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Astron. Astrophys. 318, 931-946 (1997)

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The H II/molecular cloud complex W3 revisited: imaging the radio continuum sources using multi-configuration, multi-frequency observations with the VLA

A.R. Tieftrunk 1, R.A. Gaume 2, M.J Claussen 3, T.L. Wilson 1, 4 and K.J. Johnston 2

1 Max Planck Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
2 United States Naval Observatory, 3450 Massachusetts Ave NW, Washington DC 20392-5420, USA
3 National Radio Astronomy Observatory, P.O.Box 0, Socorro NM 80375, USA
4 Astronomy Department, University of Illinois, 1002 W. Green St., Urbana IL 61801, USA

Received 28 March 1996 / Accepted 6 June 1996


The H II /molecular cloud complex W3 has been imaged at 4.9 GHz, 14.9 GHz and 22.5 GHz in the radio continuum and the H66 [FORMULA] radio recombination line with subarcsecond angular resolution using combined multiple VLA configurations. Several hypercompact continuum sources with diameters [FORMULA] AU have been detected toward IRS 4 and IRS 5, reminiscent of the "Orion Radio Zoo". They have been imaged with a maximum angular resolution of [FORMULA]. From the flux densities at 1.3 cm, 2 cm and 6 cm we determine their spectral indices. The compact and ultracompact H II regions with diameters [FORMULA] AU exhibit spectral indices [FORMULA] in the range -0.1 to [FORMULA]. The gradients in spectral indices across these regions correspond to asymmetries in their radio continuum intensities as well as gradients in the densities of the surrounding molecular gas. This indicates gradients in the electron density.

From a direct comparison of the continuum emission of the ionized hydrogen gas with the emission of the dense molecular gas and dust continuum, we refine the analyses of the interaction of the radio components with the molecular gas. From our H66 [FORMULA] recombination line data we compare the radial velocities of the H II regions with those found for the molecular gas toward W3. We find linewidth and velocity gradients in the ionized gas which are indicative of expansion and turbulent flows caused by pressure gradients in the ambient neutral gas. We propose that the observed morphologies of compact and ultracompact H II regions are determined by turbulent expansion of the ionized gas into highly anisotropic and clumpy molecular gas. Thus, we believe there can be no definite prediction for the morphologies observed in H II regions with current kinematic models without considering these inhomogeneities. Furthermore, we propose that the spatial and kinematic relation of the compact, ultracompact and hypercompact radio continuum regions toward W3 is indicative of sequentially triggered star formation caused by the pressure of the expanding H II regions and the subsequent compression of the molecular gas.

Key words: ISM: H ii regions – ISM: clouds – ISM: individual W3 – ISM: kinematics and dynamics – radio lines: ISM – stars: formation

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998